This invention relates to liquid scintillation detectors. In particular, it relates to the detection of multiple phases in liquid scintillation samples. The invention is concerned with a method for such detection and apparatus employing the method to detect the presence of multiple phases in a liquid scintillation sample.
A sample prepared for liquid scintillation counting includes a radionuclide sample matrix placed in a liquid scintillation cocktail. An electron is emitted directly from the radionuclide or is produced indirectly by the radionuclide. The electron interacts with molecules in the cocktail by mechanisms which produce one or more photons per electron event. The intensity of the light, or equivalently the number of photons produced, is proportional to the kinetic energy of the electron.
In an idealized measurement there are no interferences. Thus, for each emitted electron, one light burst or event occurs. In such a case, a count of the number of light events is a count of the number of electrons produced. This, in turn, provides a count of the number of radionuclides present, and that is the ideal purpose. A count of the actual number of electron producing nuclides is the number of disintegrations occurring. If counted for one minute, it is the disintegrations per minute (DPM).
For a variety of reasons, the real measurement produces fewer counts per minute (CPM) than actually occur. In other words, the efficiency of the process, E, is less than 100% and is given by the equation ##EQU1## where CPM is the observation provided from the actual DPM occurring. Common knowledge within this field provides means to obtain the efficiency E. As the measurement procedure itself provides CPM, the above equation gives DPM.
In practice, it frequently happens that a liquid scintillation sample separates into two or more phases, for example an organic phase and an aqueous phase. Consequently, the molecules containing the radionuclide are distributed between the aqueous and organic phases. These two phases do not, in general, exhibit the same counting efficiency. Phase 1 has efficiency E.sub.1 and produces CPM.sub.1 countable events. Phase 2 has efficiency E.sub.2 and produces CPM.sub.2 countable events. This means that the number of disintegrations, DPM, is given by: ##EQU2##
The system, however, does not provide CPM.sub.1 and CPM.sub.2, but the total CPM. Neither are E.sub.1 and E.sub.2 known in general. Therefore, DPM cannot be obtained. Thus, samples containing two or more phases can not provide information about DPM.
Multi-phase samples, namely samples having two or more phases should ideally be eliminated from the measurement system. Such multi-phase or multiple phase samples in a clear pyrex vial can usually be observed by the operator and therefore can often be removed from a sample set associated with a measurement procedure. Observation through plastic vials however, varies from difficult to impossible. Also a multi-phase condition may occur belatedly, namely after several hours. Such a condition can occur hours after being placed in a liquid scintillation counter and result in inaccuracy. For these reasons, at least, there is a need to provide a detection method and apparatus to detect multiple-phase samples.
A previous technique is that of using a graphical plot between two different quench monitors using standard one phase samples. E. T. Bush, Int. J. Appl. Rad. Isot. 19, 447 (1968), "A Double Ratio Technique as an Aid to Selection of Sample Preparation Procedures in Liquid Scintillation Counting." One of the quench monitors measures the external standard channels ratio ("ESCR") which is independent of radionuclide and characterizes cocktail quenching. The second monitor is a sample channels ratio ("SCR") which depends upon the sample spectrum and characterizes sample counting efficiency. An unknown sample with two or more phases would deviate from the plot for the one phase sample and thereby be identified. With only a single phase present in the unknown sample, then the cocktail efficiency and sample efficiency would follow the dependency of the graphical plot of the one phase ESCR against SCR. If all of the sample is either not in solution or alternatively there is multiple-phase, then the SCR value of the unknown sample will not follow the ESCR value of the plot.
A variation of this graphical plot technique is disclosed by Everett (U.S. Pat. No. 4,555,629). This procedure plots the end point of the sample spectrum rather than SCR, against the mean point of the external standard spectrum, rather than the ESCR. Multi-phase samples deviate from this graphical plot and are thereby detected.
A third approach is disclosed by Horrocks (U.S. Pat. No. 4,315,151). This approach depends upon irradiation of the sample by an external standard, for example Cs-137, to produce a Compton spectrum. A single phase sample produces one Compton spectrum; a two phase sample, two Compton spectra, etc. This multi-phase monitor consisted of plotting the inflection point of the least quenched phase, its H# (Horrocks, U.S. Pat. No. 4,075,480), against the mean of the entire Compton spectrum. Significant deviations on unknowns from this plot indicate the presence of multi-phase samples.
Difficulties exist with these prior art techniques. Should a sample spectrum be used, then the count time must increase as sample activity decreases. Accordingly, sample throughput is affected and the effective quench range is small. The Bush and Everett techniques have these disadvantages. The Horrocks system is limited where plastic vials are used in that the effective quench range is small. Also, the inflection point of the least quenched phase is needed, and this depends upon the vial composition, sample volume and the volume of each phase. Moreover, samples which contain color can be incorrectly characterized as multi-phase by the above procedures.
There is accordingly a need for the speedy detection of samples with two or more phases over a large quench range. In particular the time period should be no longer than that required to obtain an external standard quench value. There is also a need for this detection to be independent of vial composition and volume, cocktail type, and the sample or phase volume over a range, for instance, between 0.5 and 20 ml.